The present invention relates to a silver halide photographic material. More particularly, the invention relates to a silver halide photographic material which contains a cyan coupler having high resistance to discoloration in the dark and which is also improved in terms of color formation and resistance to discoloration under light.
The mechanism behind the formation of dye images in a silver halide color photographic material is that an aromatic primary amine developing agent is oxidized while reducing silver halide grains in the exposed photographic material and the resulting oxidized product reacts with a coupler already present in the silver halide color photographic material so as to form a dye. Color reproduction in this case depends commonly on the subtractive process using three couplers which respectively form yellow, magenta and cyan dyes.
There are several requirements that must be met by the couplers: first, they must have high solubility in high-boiling organic solvents, and they should be highly dispersible in silver halide emulsions and the prepared dispersion should remain stable without causing the precipitation of the couplers; secondly, the couplers should have good spectral absorption characteristics and improved tones, so that they provide sharp dye images in a wide color reproduction region; and thirdly, the couplers should produce dye images which are highly resistant to light, heat and moisture. Cyan couplers are particularly required to have a good balance among their dye image keeping properties including resistance to light, heat and moisture.
U.S. Pat. No. 2,801,171 discloses 6-[.alpha.-(2,4-di-tert-amylphenoxy)butaneamido]-2,4-di-chloro-3-methylphe nol. This cyan coupler produces an image having high resistance to light discoloration, but its resistance to dark discoloration is low. Furthermore, this coupler depends highly on benzyl alcohol for color development and is not suitable for the purpose of non-polluting processing.
Typical known cyan couplers are 2,5-diacylaminophenol compounds having an acylamino group as a substituent on the 2- and 5-positions of the phenol ring, and cyan couplers of this type are shown in U.S. Pat. No. 2,895,826, as well as in Japanese Unexamined Published Patent Application Nos. 112038/1975, 109630/1978 and 163537/1980. Such 2,5-diacylaminophenolic cyan couplers are extensively used since they produce cyan dye images having high resistance to dark discoloration. However, the resistance to light discoloration of the produced dye image is extremely low, and the unreacted cyan coupler is highly likely to turn yellow upon exposure to light (this phenomenon is hereunder referred to as Y stain under light).
U.S. Pat. No. 3,772,002 discloses 6-[.alpha.-(2,4-di-tert-amylphenoxy)butaneamido]-2,4-di-chloro-3-ethylphen ol as a coupler capable of producing an image having high resistance to dark discoloration, but it still depends highly on benzyl alcohol for color development and if it is processed with a color developer containing no benzyl alcohol, sufficient color formation is not ensured. This coupler also has low resistance to light discoloration.
The present inventors previously found that if a phenolic cyan coupler is dispersed in a silver halide emulsion with the aid of a high-boiling organic solvent having a dielectric constant of 6.1 or more (e.g. dibutyl phthalate), the color formation resulting from processing with a color developer containing no benzyl alcohol can be improved. While this method is fairly effective in improving the color formation on a photographic material processed with a benzyl alcohol free color developer, Y stain under light (yellowing of the white background which is presumably due to the presence of the unreacted cyan coupler) is increased to an undesired degree.